As the information industry develops, larger information treatment may be required. Therefore, the demand for a data storage medium that can store a large capacity of information continuously increases. Due to this increased demand, studies about smaller information storage mediums having higher data storing speeds have been conducted, and as a result, various kinds of information storage apparatuses have been developed.
Information storage apparatuses may be divided into volatile information storage apparatuses and non-volatile information storage apparatuses. In the case of volatile information storage apparatuses, when power is turned off all recorded information may be erased. However, volatile information storage apparatuses may have a higher information recording and/or reproducing speed. In the case of non-volatile information storage apparatuses, recorded information may not be erased even if power is turned off.
An example of a volatile information storage apparatus is a dynamic random access memory (DRAM) device. Examples of non-volatile information storage apparatuses may include a hard disc drive (HDD) and a random access memory (RAM) device.
FIG. 1A is a cross-sectional view of a magnetic random access memory (MRAM), which is an example of a related art non-volatile information storage apparatus.
Referring to FIG. 1A, a magneto-resistive structure 14 may be formed on a base electrode 12, which may be electrically connected to a transistor 10. A first conductive line 16a may be formed on a lower side of the base electrode 12 in a region corresponding to the magneto-resistive structure 14. A second conductive line 16b may be formed on the magneto-resistive structure 14. In the related art MRAM, to record information, a magnetic field may be applied to the magneto-resistive structure 14 to magnetize a magnetic material of the magneto-resistive structure 14 into one of two memory states.
For example, when a current flows in the first conductive line 16a or second conductive line 16b, a magnetic field may be generated around the first conductive line 16a or the second conductive line 16b. The generated magnetic field determines the magnetization direction of a free layer 104, of the magneto-resistive structure 14, and thus, information may be recorded. The magneto-resistive structure 14 may include an anti-ferromagnetic layer 101, a fixed layer 102 whose magnetization direction is fixed by the anti-ferromagnetic layer 101, a non-magnetic layer 103 formed on the fixed layer 102, and/or the free layer 104 which is formed on the non-magnetic layer 103 and whose magnetization direction may be reversed.
A memory device that uses a magnetic field to switch the direction of a desired memory cell may have one or more of the following problems.
First, when the size of a unit cell is reduced to realize a higher density memory device, the coercivity of the free layer 104 may be increased, thereby increasing a switching field of the memory device. Accordingly, the magnitude of a current to be applied to the memory device may have to be larger. Second, in a memory device that includes a plurality of memory cells, the memory cells around the first conductive line 16a and the second conductive line 16b may be affected by the magnetic field and undesired memory cells may also be switched. As a result, in the magnetic memory device that uses the magnetic switching method, there may be difficulty in ensuring selectivity and/or higher density. To address the problems described above, a magnetic memory device that uses a current induced switching (CID) method has been studied.
FIG. 1B is a perspective view of a related art memory device that uses the CID method. Referring to FIG. 1B, a magneto-resistive structure 14 and an upper electrode 18 may be sequentially formed on a lower electrode 17, which is electrically connected to a transistor 10. An anti-ferromagnetic layer 101, a fixed layer 102 whose magnetization direction is fixed by the anti-ferromagnetic layer 101, a non-magnetic layer 103, and/or a free layer 104 which may be formed on the non-magnetic layer 103 and whose magnetization direction can be reversed may be sequentially formed on the magneto-resistive structure 14 that corresponds to the lower electrode 17.
In the CID method of FIG. 1B, the free layer 104 may be directly switched to a desired direction by using spin transfer of electrons by applying a current whose spin is polarized in one direction to the magneto-resistive structure 14 through the lower electrode 17.
This method may be advantageous for realizing higher density because as the cell size is reduced, the required current may be reduced. An issue of the magnetic memory device that uses the CID method is that a critical current required for switching may still be too large to commercialize. Thus, studies for reducing the critical current are in progress.
Methods of reducing the critical current that have been proposed will now be described.
First, the reduction of a polarization factor may reduce the critical current. As the polarization factor of an inputted current becomes larger, the critical current required for switching may be reduced. However, because the polarization factor is a basic property of a substance, the polarization may be slightly increased. Second, a method of using a multi-layer structure has been proposed; however, this process is complicated. Third, studies have been conducted about using a half-metal because it is known that the half-metal theoretically has a large polarization factor. However, a half-metal may still be difficult to apply to products. Fourth, a method of reducing the value of remnant magnetization and thickness of a magnetic layer has been proposed. However, due to the superparamagnetic effect caused by the reduction of the volume of cell itself, the stability of recorded information may not be ensured.